1. Field of the Invention
The present invention relates to a current balance circuit for use in a switching power supply unit, adapted to control a balance between a current of the switching power supply unit and a current of one or more switching power supply units connected in parallel to the switching power supply unit so as to apply a stable voltage to a load.
2. Description of the Related Art
It was common to use a single power supply unit in order to supply a stable operating voltage to various types of electronic apparatuses, such as computers. Recently, however, the use of plural small power supply units operating in parallel has been increasing in view of size reduction and improved reliability of the power supply units. In general, switching power supply units are used as the small power supply units so as to stabilize output voltage. The use of the small switching power supply units operating in parallel allows stable voltage supply to a load even when one of the switching power supply units fails. This is because the other switching power supply units can make up for the failed switching power supply unit. The size reduction and reliability improvement of the power supply units are thus achieved.
FIG. 4 shows a configuration of a system in which n units of switching power supply units CONV1, CONV2, . . . CONVn are connected in parallel so as to apply a stable voltage to a load. In the case where one of the switching power supply units becomes unable to supply power to the load due to a failure, n-1 units of the switching power supply units continue to supply power. If the switching power supply units CONV1–CONVn have capacity to supply more power, the switching power supply units operating normally can maintain the power supply to the load even when two or more switching power supply units fail. In FIG. 4, Vin+ and Vin− indicate input voltages, Vout+ and Vout− indicate output voltages, and CB represents a current balancing terminal. The current balancing terminals CB are connected to each other so as to compare output current values of the switching power supply units CONV1–CONVn, thereby balancing output currents of the switching power supply units CONV1–CONVn.
FIG. 5 is a block diagram illustrating a related-art switching power supply unit. The power supply unit comprises a transformer 51, a rectifier circuit 52, a smoothing circuit 53, an output voltage detection circuit 54, an output current detection circuit 55, a current balancing circuit 56, a control circuit 57, a switching transistor Q1, and a current balancing terminal CB. Vin and Vout indicate an input voltage and an output voltage, respectively. When two or more of such a switching power supply unit operate in parallel, the current balancing terminals CB of the power supply units are connected to each other as described above.
The control circuit 57 controls switching operations of the switching transistor Q1 connected to a primary winding of the transformer 51. An induced voltage generated in a secondary winding of the transformer 51 is rectified by the rectifier circuit 52, smoothed by the smoothing circuit 53, and then output as the output voltage Vout. The output voltage Vout is applied to a load. The output voltage detection circuit 54 detects the output voltage Vout so as to input the output voltage Vout to the control circuit 57. The control circuit 57 controls the period during which the switching transistor Q1 is turned on so as to adjust the output voltage Vout to a predetermined level, thereby stabilizing the output voltage Vout. Also, a detection value detected by a current transformer or a resistor is input to the output current detection circuit 55 such that the output current detection circuit 55 detects an output current and inputs an output current detection signal to the current balancing circuit 56. Information about output currents of other switching power supply units operating in parallel are input to the current balancing circuit 56. The current balancing circuit 56 inputs a control signal for the output current to the control circuit 57 so as to balance the output currents of the switching power supply units.
FIG. 6 is a circuit diagram illustrating the current balancing circuit 56. The current balancing circuit 56 comprises resistors R1–R4, Ra, and Rb, a diode D1, and an operational amplifier OPA. The output current detection circuit 55 of FIG. 5 inputs the output current detection signal to the diode D1. The operational amplifier OPA inputs the control signal to the control circuit 57. The output current detection signal is input to the resistor Rb via the diode D1, so that an output current detection voltage V1 corresponding to the output current detection signal is generated across the resistor Rb. The output current detection voltage V1 is applied to a negative terminal of the operational amplifier OPA via the resistor R2, and is also applied to a positive terminal of the operational amplifier OPA as a voltage drop across the resistor R3 formed by the resistors Ra, R1, and R3. The current balancing terminal CB is connected to a connection point between the resistors Ra and R1. It is to be noted that the relationship between resistances of the resistors R1, R3, Ra, and Rb connected to the positive terminal of the operational amplifier OPA is R1+R3>Ra+Rb. Accordingly, the operational amplifier OPA inputs the difference between the output current detection voltage V1 and an output current detection voltage from the other current balancing circuits to the control circuit 57.
When two or more of the above described switching supply units operate in parallel with each other, the current balancing terminals CB of the switching power supply units are connected in parallel. If one of the switching power supply units has a higher output current detection voltage than the other, a signal for making the lower output current detection output voltage follow the higher output current detection voltage is input to a control circuit 57 from an operational amplifier OPA in the switching power supply unit having the lower output current detection voltage so as to balance the currents. In the switching power supply unit having the higher output current detection voltage, a signal for reducing the difference between the voltages to fall below a predetermined value is input to a control circuit 57 from an operational amplifier OPA, thereby balancing the currents. That is, each of the switching power supply units adjusts the output currents to the same level, i.e., controls the current balance.
There is known a current balancing circuit as disclosed in, for example, Patent Document 1. According to Patent Document 1, the current balance is controlled by detecting currents of primary sides of transformers of plural switching power supply units, whose current balancing terminals are connected to each other, and then integrating the difference between values of the detected currents of the primary sides of the switching power supply units. There is also known a configuration as disclosed in, for example, Patent Document 2. According to Patent Document 2, currents of secondary sides of transformers of plural switching power supply units are detected. Current balancing terminals of the switching power supply units are connected to each other so as to limit the difference between the values of the detected currents of the secondary sides of the transformers of the switching power supply units, thereby preventing a sudden change in output current.
<Patent Document 1> Japanese Examined Patent Publication No. 6-26473
<Patent Document 2> Japanese Patent Laid-Open Publication No. 2001-161063
Referring back to FIG. 6, as the current balancing terminal CB of the switching power supply unit is connected in parallel to a current balancing terminals CB of other switching power supply units as shown in FIG. 4, the resistors Ra and Rb of the switching power supply units are connected in parallel to each other via the current balancing terminals CB. If one of the switching power supply units fails, the output current thereof becomes zero, so that the voltage V1 applied to the resistor Rb via the diode D1 of the current balancing circuit of the failed switching power supply unit becomes zero. As a result, electric potentials of the current balancing terminals CB of the current balancing circuits of the other switching power supply units are reduced by the resistors Ra and Rb connected in parallel, so that impedance of a current balance control system during normal operations is lowered, thereby significantly lowering the current balance properties.
For example, with reference to FIG. 7A, if one of the switching power supply units is a master power source (switching power supply unit whose output current detection voltage is slightly higher than output current detection voltages of the other switching power supply units), the other switching power supply units are slave power sources. The master power source and the slave power sources maintain balance between the currents in accordance with the increase of a load current, while operating with a predetermined current difference. However, if one of the switching power supply units fails, resistors Ra and Rb of the failed switching power supply unit are connected in parallel to resistors Ra and Rb of the other switching power supply units via the current balancing terminals CB connected in parallel as mentioned earlier, so that impedance of a current balance control system is lowered. Therefore, output currents of the slave power sources follow the increase of the output current of the master power source with delay, resulting in increasing the difference between the output currents. That is, current balance properties are lowered.